1. Field of Invention
The present invention relates to a semiconductor device in which an electric field effect type transistor, such as MIS (Metal-Insulator-Semiconductor) type transistor or an MOS (Metal-Oxide-Semiconductor) type transistor, is formed on a substrate, an electro-optical device holding an electro-optical material using such a semiconductor device, an electronic apparatus using such an electro-optical device, and to a method for manufacturing the semiconductor device.
2. Description of Related Art
In an electro-optical device, such as an active-matrix-type liquid crystal device or an organic electroluminescent display device, a substrate on which a plurality of thin film transistors (electric field effect transistors, hereinafter referred to as TFTs [Thin Film Transistors]) are formed as active elements for switching pixels, is used.
There are two possible related art structures of the TFT formed on such a substrate: a self-aligned structure shown in FIG. 21(A) and an LDD structure shown in FIG. 21(B).
In the self-aligned-structure TFT, parts of a source region 420 and a drain region 430 facing both ends of a gate electrode 460, with a gate insulating film 450 interposed therebetween, are heavily doped as shown in FIG. 21(A). As a result, there is an advantage that the level of ON current is high as shown with a dotted line L2 in FIGS. 2 and 6.
However, in the self-aligned-structure TFT, since the electric field in one end of the drain is intense, there are problems that the level of OFF leakage current is high and the current level increases steeply as shown with a dotted line L2 in FIGS. 2 and 6.
In contrast to the self-aligned-structure TFT, in the LDD-structure TFT shown in FIG. 21(B), a source region 420 and a drain region 430, facing both ends of a gate electrode 460 with a gate insulating film 450 interposed therebetween, contain a lightly doped source region 421 and a lightly doped drain region 431, respectively. Thus, in the LDD-structure TFT, since the electric field in one end of the drain declines, the level of OFF leakage current is low, and the sudden rise of current level is reduced or prevented, as shown with a one-dot-chain line L3 in FIGS. 2 and 6 (See M. Yazaki, S. Takenaka and H. Ohshima: Jpn. J. Appl. Phys. vol. 31 (1992) Pt. 1, No 2A pp. 206-209).
However, in the LDD-structure TFT, since the lightly doped regions are interposed between the source region 420 and the drain region 430, there is a problem that the level of ON current is low, as shown with a one-dot-chain line L3 in FIGS. 2 and 6. Furthermore, in the LDD-structure TFT, if the dimensions of the lightly doped source region 421 and the lightly doped drain region 431 are lengthened in order to further decrease the OFF leakage current, there is a problem that the ON current is markedly reduced.
As described above, in related art structures, there is a problem that an improvement of one characteristic results in a detriment to another characteristic because of a trade-off relationship between the properties of ON current and OFF leakage current.